The dielectric ceramic has been widely utilized for a dielectric resonator and MIC dielectric substrate etc. for use in a high frequency region including microwave and/or millimeterwave regions.
As the dielectric ceramic for such a purpose, a variety of materials such as those belonging to ZrO.sub.2 --SnO.sub.2 --TiO.sub.2 type, Ba.sub.2 Ti.sub.9 O.sub.20, (Ba,Sr) (Zr,Ti)O.sub.3 type or BaO.ZnO.Ta.sub.2 O.sub.5 type etc. are known.
Although these materials have superior characteristics in microwave energy of a frequency of around 10 GHz, and having a dielectric constnat (.epsilon.r) of 20 to 40, a Q of 2,000 to 6,000 and a temperature coefficient of resonant frequency (.tau..sub.f) of around 0 ppm/.degree.C. with the recent use of higher frequencies, it has been desired to provide a ceramic having a higher Q value.
For example, materials belonging to Ba(Zn,Ta)O.sub.3 type are described in Japanese Patent Application (OPI) No. 35454/78 in detail (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). According to this reference a dielectric ceramic disc having a diameter of 5 mm and thickness of 2 mm obtained by sintering the disc at 1,360.degree. to 1,460.degree. C. in air for two hours has a dielectric constant (.epsilon.r) measured from a resonant frequency (around 11 GHz) and the dimensions the unloaded Q measured by using the band reflection method and the temperature coefficient (.tau..sub.f) of resonant frequency measured in a range -30.degree. C. to +70.degree. C., 25 to 30, 3,520 to 3,730 and 5 to 20, respectively.
The crystal structure of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 is disclosed in F. Gallasso and J. Pyle "Ordering in Compounds of the A(B'.sub.0.33 Ta.sub.0.67)O.sub.3 type", Inorganic Chemistry, vol. 2, No. 3, pages 482-484 (1963). The material is a compound with a unit cell having a perovskite structure of the ABO.sub.3 type. Zn and Ta are each B site ions and are ordered to form a hexagonal superlattice.
FIG. 1 shows the superlattice structure of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 which is similar to Ba(Sr.sub.1/3 Ta.sub.2/3)O.sub.3 disclosed in the above-mentioned paper. In FIG. 1, Zn(1) and Ta(2) in the B site ions are ordered and in a ratio of 1:2. In the same figure reference numerals 3 and 4 depict Ba and O, respectively.
On the other hand, according to "Microwave Dielectric Materials and their Applications," the Annual Report of Study Group for the practical use of BaTiO.sub.3 material vol. 30 xxx-164-1036, Sept. 11, 1981, the ordered structure of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 may depend largely upon the sintering conditions. According to the report, the ordered arrangement of Zn and Ta can not be achieved, when sintered at 1,350.degree. C. for about 2 hours, thus failing to improve Q value. When sintered at the same temperature for 120 hours the ordered arrangement is realized with the Q value being 14,000. Further, when the material is sintered at 1,650.degree. C. for 2 hours, the Q value thereof becomes 10,000 to 11,000. This is consistent with the data of FIG. 2, which shows the dependency of an X-ray diffraction pattern in the superlattice with sintering time.
In the first case, i.e. when the sintering temperature is 1,350.degree. C., it is necessary to maintain the temperature for a time period as long as 120 hours to obtain the ordered arrangement of Zn and Ta, which is a considerable obstacle in improving productivity and leads to increase of manufacturing cost. Therefore this can not be used in the industrial scale.
In the case where the material is sintered at the higher temperature, it is possible to obtain the ordered arrangement of Zn and Ta for a relatively short time. However, since there is evaporation of Zn at such high temperatures, it is impossible to obtain a dense ceramic. That is, the density is decreased, for example, from 7.7 g/cm.sup.3 to 6.5 g/cm.sup.3. Therefore, the resultant ceramic cannot be used with sufficient reliability in humid environments. Further the sintering furnace itself must be designed specially to accomodate such high temperatures and requires a large amount of energy to maintain the high temperature.